Induction heating device, cooking appliance using such device and method for assembly thereof

ABSTRACT

An induction heating device and method are disclosed. The device may include a support plate and a coil assembly and a plurality of ferrite bars or the like interposed between the coil assembly and the support plate. Additionally, a central polymeric element may be directly in contact with and interposed between the coil assembly and the support plate. The central element may have a plurality of radial seats configured for engagement with at least one end of the ferrite bars, the seats are configured to locate the bars in a predetermined position.

TECHNICAL FIELD

The present invention relates to an induction heating device comprisinga support plate and a coil assembly, between the coil assembly and thesupport plate being interposed a plurality of ferrite bars. With theterm “ferrite bars” we mean any kind of elongated magnetic fieldconcentrator located beneath the coil assembly.

BACKGROUND

The above heating devices are used to heat and cook food thanks to heatenergy generated in a ferromagnetic container placed above the coilassembly and supported on a ceramic glass or the like.

The known induction heating devices consist of a series of layersdedicated to different functions. All of them are composed of differentmaterials and geometries. In some solutions layers are connected withglue. All the parts (with the exception of the support plate, usuallymade of aluminum) have a hole in the center which is designed to allowthe insertion of a sensor holder for a thermal sensor contacting theceramic glass. On the top of the last layer, i.e. the coil assembly,there is a thermal insulating layer for instance of rock wool. Thesequence of the layers, starting from the top is: rock wool>glue>coppercoil>glue>mica>glue>ferrite>glue>aluminum base. Mica and copper coil areusually supplied as a single assembly.

Other heating induction devices, for instance as described in EP0713350and EP1560462, comprise a disk-shaped plastic support interposed betweenthe support plate and the coil assembly and having a plurality ofhousings for containing the ferrite bars. Also in these solutions glueis used for holding together the different layers.

All the above known solutions present drawbacks, either in view of theextensive use of glue which makes the assembly process quite complex andunreliable in terms of controlled final dimensions, or in view of theincrease of cost due to the use of quite complex and large components asthe disk-shaped plastic supports. Another problem is linked to the needof having, in an automated assembly process, a reliable and constantthickness of the flat induction heating device, particularly because thequantity of glue cannot always being dosed in a constant manner.

SUMMARY

It is an object of the present invention to provide an induction heatingdevice which does not present the above drawbacks and which is easy andeconomical to be produced.

According to the invention, the above object is reached thanks to thefeatures listed in the appended claims.

The technical solution according to the present invention comprises asmall and simple plastic central component that exploits the symmetricgeometry of the coil assembly in order to design a core that can alsopreferably integrate the sensor holder and preferably connects thedifferent layers with a quick mechanical snap-engagement fasteningsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features according to the present invention willbe clear from the following detailed description, provided as a notlimiting example, with reference to the attached drawings, in which:

FIG. 1 is a perspective view of a key component of the induction heatingdevice according to the invention;

FIG. 2 is an exploded view, partially cross sectioned, of an inductionheating device according to the invention, which includes component ofFIG. 1;

FIG. 3 is a perspective view from the bottom of the induction heatingdevice of FIG. 2, in an assembled configuration;

FIG. 4 is an enlarged perspective view of a central portion of thedevice of FIG. 2, in an assembled configuration,

FIG. 5 is a schematic top view of how the key component of FIG. 1 isused for fitting the ferrite bars; and

FIGS. 6 and 7 are schematic views similar to FIG. 5 showing differentsolutions for different layouts for ferrite bars position according todifferent coil sizes.

DETAILED DESCRIPTION

With reference to the drawings, the induction heating device comprises acentral component K composed of a co-injection of two differentmaterials. With reference to FIG. 1, the component K comprises a plasticbody 2 of thermoplastic material, on which is co-injected a central partof rubber, the sensor holder 1. The plastic body 2 is made ofthermoplastic or thermosetting polymer having a high Young's modulus.

The shape of the component K recalls that of a snowflake, since itcomprises a central portion 10 shaped as a regular polygon, for instancean hexagon (where the rubber or similar elastomeric material iscentrally co-injected), on whose apexes 10 a are integrally formed otherregular auxiliary polygons 12, in the shown example triangles.

Such shape is due to the main technical purpose of the component K, i.e.to constrain the ends of ferrite bars 4 in the position required tochannel or concentrate the electromagnetic field. Between each side 10 bof the hexagon defining the central portion 10 of the component K andtwo facing sides 12 a of two adjacent auxiliary triangles 12 it isdefined a quadrangular seat 14 (open on top and bottom) for an end 4 aof a ferrite bar 4. In the example shown in FIGS. 1, 2 and 4, thecentral component K defines six seats 14 for the ferrite bars, but ofcourse it can be formed with a different number of seats (as shown inFIGS. 6 and 7). The thickness of the portion 10 corresponds to thethickness of the ferrite bars 4, so that all such components can besandwiched between other components of the induction heater (as it willbe clear from the following description) in a very precise way in termsof final thickness. Moreover, the dimension of the seats 14 (whichallows the assembly of the ferrite bars 4 with a predetermined degree ofinterference), together with the stiffness of the thermoplastic orthermosetting material allows a very stable position of the ferrite bars4, despite the forces acting on them during normal operation.

The sensor holder 1 is a co-injection of rubber, with a total heightslightly higher than the thickness of the central portion 10, becausethe holder 1 needs to generate a spring effect to keep a sensor 8 (FIGS.2 and 4) pressed on the bottom face of the glass (not shown) of theinduction cooktop.

The central portion 10 of the component K is also provided with aplurality of elastic hooks 16 and 18 which are oriented parallel to thecentral symmetry axis of the component K. A first crown of upper hooks16 can be seen in FIGS. 1 and 2, while a second crown of lower hooks 18can be seen in FIG. 3.

FIG. 2 shows the entire induction heater assembly with the centralcomponent K. A copper coil 6 is stuck on a mica layer 5 with glue andthey form a single component. The coil is assembled in the followingsequence: the central component K is connected to an aluminum base orplate 3 through the lower crown of hooks 18 with a light pressure inorder to allow a snap-engaging action of the hooks 18 on a circular edge22 a of a central hole 22 of the base 3 (FIG. 3). The central hole 22with its edge 22 a can be replaced by small slots (not shown) which canbe snap-engaged by the lower crowns of hooks 18 for reaching the sametechnical result. The dimension of the elastic hooks 18 is selected inorder to fix in a stable way the component K on the aluminum base 3without the use of any glue. At this point of the assembly process, theferrite bars 4 are collocated with interference in the defined seats 14,obtaining a configuration (shown in FIG. 5) in which the ferrite bars 4are radially centered on the component K as rays of a star. Then themica layer 5 and the copper coil 6 assembly is positioned on the centralcomponent K and with a little pressure (similarly to what already donefor the base 3), connected with the upper hooks 16 so that theysnap-engage with a circular edge 24 a of a central hole 24 of the coilassembly, visible in FIG. 4. The component K is interposed and incontact with the aluminum base 3 and with the coil assembly,particularly with the mica layer 5 thereof.

A thermal insulation layer 7, for instance of rock wool, is then placedon the copper coil 6 and a temperature sensor 8, for instance a NTCsensor, is joined to the central component K through its insertion incorresponding joints 26 (FIG. 4) on the sensor holder 1 of the centralcomponent K, so that a mechanical constraint of the rock wool 7 isobtained. The rock wool layer 7 is preferably provided with a centralhole 7 a so that the sensor 8 is interposed between the sensor holder 1and the ceramic glass of the induction cooktop.

The embodiment shown in FIG. 6 differs from the above in the differentnumber of ferrite bars (eight instead of six) in order to match adifferent size of the coil and in the different shape of the centralcomponent K′ (octagonal and not hexagonal). Moreover the ferrite bars 4and 40 have two different dimensions, with the longer bars 4 closer tothe sensor holder 1.

In the embodiment shown in FIG. 7 (for a coil having a large size) thecentral component K″ has an overall pentagon shape and it is adapted tohouse ten ferrite bars 4 and 42, also in this case of different length.

The solution according to the invention, independently on whichembodiment is used, has many benefits in terms of cost reduction andimproved assembly procedure.

First of all, it is possible to get rid of the glue required, in theknown solution, to position the ferrite bars on the aluminum base.Accordingly, there is also a decrease of the assembly time of theinduction heating device and of the ferrite bars, by replacing the gluewith snap-engaging fastener as hooks integral with a central simplecomponent. There is a more accurate positioning of the ferrite bars,with a reduction in position variability caused by the unreliablequantity (and thickness) of the glue, and therefore a better control ofthe electromagnetic field in the working conditions of the inductionheating device. It is also possible to avoid the use of glue required toposition the mica layer on the ferrite, with a decrease of connectiontime of the mica layer by replacing the glue with snap-engagingfasteners.

Last but not least it is possible to easily integrate the temperaturesensor holder with a decrease of the number of components, increasingthe stability of the sensor holder because it is no longer connectedwith fasteners, but it is part of a single body.

The invention claimed is:
 1. An induction heating device, comprising: asupport plate; a coil assembly; a central element interposed between thecoil assembly and the support plate, the central element having aplurality of radial seats; and at least one ferrite bar, wherein each ofthe radial seats is dimensioned to receive an end of the at least oneferrite bar with a predetermined degree of interference so that the atleast one ferrite bar extends radially from and centered on the centralelement between the coil assembly and the support plate.
 2. Theinduction heating device according to claim 1, wherein the radial seatsare defined by radial extensions of the central element.
 3. Theinduction heating device according to claim 1, wherein the centralelement is made, at least partially, of a polymeric material.
 4. Theinduction heating device according to claim 3, wherein the centralelement comprises a central portion made of elastomeric material,wherein the central element and the central portion are co-injected andadapted to urge a sensor against a glass surface of a cooktop in aworking condition of the heating device.
 5. The induction heating deviceaccording to claim 1, wherein the central element includes a pluralityof hook-shaped fastening portions adapted to cooperate with the supportplate in order to snap-engage the central element to such plate.
 6. Theinduction heating device according to claim 1, wherein the centralelement presents a plurality of hook-shaped auxiliary fastening portionsadapted to cooperate with an edge of a central hole of the coil assemblyin order to snap-engage the central element to such assembly.
 7. Theinduction heating device according to claim 1, wherein the centralelement presents a flat central portion having a polygonal shape and athickness corresponding to thickness of the ferrite bars, each apex ofthe polygonal central portion having a flat radial appendix of apolygonal shape so that each side of the polygonal central portion andtwo facing sides of two adjacent appendixes define a quadrangular seatfor the end of the corresponding ferrite bar.
 8. The induction heatingdevice element according to claim 7, wherein the central portion ishexagonal and the appendixes are triangular.
 9. The induction heatingaccording to claim 1, wherein the coil assembly comprises a mica layerinterposed between the coil and the ferrite bars.
 10. The inductioncooktop according to claim 9, wherein a thermal insulation layer isinterposed between the coil assembly and the glass plate.
 11. Aninduction cooktop, comprising: at least one induction heating device; aglass support plate; a coil assembly; a plurality of ferrite bars; acentral element interposed between the coil assembly and the supportplate, the central element having a plurality of radial seatsdimensioned to receive and locate ends of the plurality of ferrite barswith a predetermined degree of interference so that the plurality offerrite bars extend radially from and centered on the central elementbetween the coil assembly and the support plate, and a temperaturesensor interposed between the central element and the glass supportplate of the cooktop.
 12. The induction cooktop according to claim 11,wherein the central element presents a seat for the temperature sensor.